I'm preparing to move from Moped to the new official Ruby driver. I've noticed it accidentally introduces new reserved field name: "ok". This line checks if the first document returned from query contains key "ok" and that its value equals 1:

https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/operation/result.rb#L208

You can successfully create documents with that field, but if you set its value to anything other than 1 and later query the collection returning that document as the first result then you get OperationFailure error with a very unhelpful empty message.

Attached are my test scripts. Here's the run log:

mongoid-errors$ruby okey.rb 
Using Mongoid 5.0.0.rc0
Saved document #<MyDocument _id: 55e21bd24164617255000000, okey: true>
Loaded: #<MyDocument _id: 55e21bd24164617255000000, okey: true>
mongoid-errors$ruby also-okey.rb 
Using Mongoid 5.0.0.rc0
Saved document #<MyDocument _id: 55e21bd5416461725a000000, ok: 1>
Loaded: #<MyDocument _id: 55e21bd5416461725a000000, ok: 1>
mongoid-errors$ruby not-ok.rb 
Using Mongoid 5.0.0.rc0
Saved document #<MyDocument _id: 55e21bdc416461725f000000, ok: true>
Mongo::Error::OperationFailure
Operation failed. Here's the useless message:
Message here >>  << end of message

Please note that MongoDB documentation does not say anything about "ok" field. Here is the section on field name constraints:

http://docs.mongodb.org/manual/core/document/#field-names